As the reserves of conventional crude oils decline, heavy oils must be upgraded to meet market demands. Crude oil is typically first processed in an atmospheric crude distillation tower to provide fuel products including naphtha, kerosene and diesel. The atmospheric crude distillation resid bottoms stream is typically taken to a vacuum distillation tower to obtain vacuum gas oil (VGO) that can be feedstock for an FCC unit or a hydrocracking unit and vacuum residue (VR).
Solvent deasphalting (SDA) generally refers to refinery processes that upgrade hydrocarbon fractions using extraction in the presence of a solvent. The hydrocarbon fractions are often obtained from the distillation of crude oil, and include hydrocarbon residues or resids or gas oils from atmospheric column or vacuum column distillation. SDA permits practical recovery of higher quality oil, at relatively low temperatures, without cracking or degradation of heavy hydrocarbons. SDA separates hydrocarbons according to their solubility in a liquid solvent, as opposed to volatility in distillation. Lower molecular weight and most aliphatic components are preferentially extracted. The least soluble materials are high molecular weight and mostly aromatic and polar components. This makes the deasphalted oil (DAO) extract light and aliphatic, and the asphaltic raffinate also known as pitch, heavy and aromatic. Suitable solvents for SDA include propane and higher molecular weight paraffins, such as butane and pentane, for example. The pitch stream generally contains metal compounds as well as high molecular weight hydrocarbons.
SDA typically recovers no more than about 40 wt % product. Hence, further recovery is very desirable in SDA to make it worthwhile. Solvent deasphalting in series has been proposed but not successfully commercialized. The second deasphalting column in the series receives a light pitch stream that has been stripped of solvent, typically. The light pitch stream extracted with a heavier solvent leaves very heavy asphaltenes in a heavy pitch stream. Once the heavy pitch stream is stripped to remove residual solvent, the heavy pitch stream is so viscous that it can set up in the heavy asphaltene stripper, making removal and transport prohibitive.
There is an ongoing need for two-stage deasphalting processes and apparatus to increase recovery of the lighter useable portion of the feed without making removal and transport of the heavy pitch stream difficult.